Daily changes in reflex ventilatory responses to hypoxia were studied in 56 young healthy men participated. A rebreathing technique--progressive isocapnic hypoxia--was used to measure hypoxic chemoreflex reactivity. The measurements were repeated every 3 hours during a 24-hour period. The ventilatory response to hypoxia--analyzed as the relationships (slope) MV/SaO2 (minute ventilation/arterial oxygen blood saturation) and P0.1/SaO2 (mouth occlusion pressure/oxygen arterial blood saturation) remained relatively stable throughout the examined period exhibiting low-amplitude differences from one another with the maximum variation at 12:00 hours.